This invention relates to a method of cutting off a masking film of a silicon wafer, and particularly to the method of cutting off the masking film automatically long the outer periphery of the silicon wafer to which the masking film was adhered before lapping or cutting the reverse side of the silicon wafer, which is, for instance, used for an IC, an LSI or a VLSI. In the case of the so-called IC manufacturing process, this method may be utilized generally in the crystalizing process including the slicing, lapping, polishing or back-grinding step.
The IC manufacturing process may be divided into the steps of the crystalizing process, the pre-wafer process, the after-wafer process and the checking process. In the crystalizing process, the slicing, lapping and polishing or back-grinding steps are carried out after crystalization of the silicon. The lapping step is aimed at removing the strained layer, which is made in the slicing step, from the surface of the silicon crystal, and also at thinning the sliced thick crystal (wafer) to a predetermined thickness and finishing the sliced surface to be a mirror finish for electric circuit formation. It is necessary that high precision be used on the lapped surface to provide flatness and smoothness.
Nowadays, in view of IC packaging, it becomes general to take a process for making a wafer thinner after the wafer forming process in order to fulfil the requirements in packaging and to solve a heat problem as well as shortening of the size of the IC chip. In the case of a CMOS and the like which normally heat less, of course, it is not necessary to thin the wafer, but it becomes necessary to thin same in order to minimize the package therefor. In the process of thinning the wafer, the reverse side of the wafer is cut and it is, of course, necessary to have a lapping or back-grinding step.
Prior to the lapping stage, the silicon wafer is waxed with its circuit face and fixed onto an internal gear table so as to lap the reverse side thereof or a masking film (for instance a laminate) is stuck to the circuit face of the silicon wafer so as to be fixed with suction force for lapping the reverse side thereof. This operation aims at protecting the circuit face of the silicon wafer in lapping, etching or backgrinding step. However, such waxing or sticking of the masking film is carried out manually. Particularly, it relies on a manual operation to cut off the outer periphery of the masking film, which extends from the silicon wafer, along the outer periphery of the silicon wafer.
According to the conventional method of waxing a silicon wafer, there are considerable disadvantages as well as being troublesome and inefficient. On the other hand, it has the same disadvantages in the method of sticking of the masking film since it is carried out manually.
Particularly, the conventional method involves not only inefficiency in the operation to cut off the masking film along the outer periphery of the silicon wafer which has to be carried out by a skilled worker, but also a disadvantage in yield rate. Namely, since the outer portion of the masking film is manually cut along the outer periphery of the silicon wafer by means of a cutter, even the skilled worker applies uneven and unnecessary pressure to the outer periphery of the silicon wafer. As the result, edge chips are made around the outer periphery so that yield rate becomes low and it is difficult to avoid mistakes and damage to the wafer. Further, when the outer periphery of the masking film is cut off, there sometimes remain chips (so-called whiskers) around the cutting section of the masking film though it would be better that the cutting section of the masking film becomes even or clean. In this case, the securing device forms the whiskers in lapping process and as the result the very expensive silicon wafer is damaged and it sometimes brings about that the diamond securing means of a lapping machine is also broken.
It is of course desirable to automate such manual cutting of the masking film, but therein lies the following problems:
(1) It cannot be avoided that accuracy of the outer diameter of the silicon wafer is beyond .+-.0.5 to a standard per each lot of the silicon wafers;
(2) the number of the positioning flats is one or two, but not constant depending on the type of the machine or the size of the wafer;
(3) there occurs edge chips with the wafers at a considerable percentage prior to adhesion of the masking film; and
(4) it is preferable to cut the wafer along the outer periphery of the wafer and therefor the above-mentioned problems 1, 2 and 3 have to be solved.